Derivative Instruments and Hedging Activities Schedule of Interest Rate Swap Receivers Associated with the Investment Portfolio (Details) $ in Thousands	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Derivative [Line Items]
Derivative, Notional Amount	$ 26,141,370	$ 26,390,596	$ 21,998,381	$ 27,401,335	$ 33,709,043	$ 48,115,523
Interest Rate Swap [Member]
Derivative [Line Items]
Derivative, Notional Amount	13,697,000	$ 15,425,513	14,268,806	$ 16,225,523	$ 19,929,000	$ 18,584,000
Interest Rate Risk Associated with the Investment Portfolio [Member] | Interest Rate Swap [Member] | Short [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 3,064,000		$ 3,164,000
Derivative, Average Variable Interest Rate	1.784%		2.145%
Derivative, Average Fixed Interest Rate	0.645%		0.431%
Derivative, Average Remaining Period Until Maturity	4.80		6.26
Interest Rate Risk Associated with the Investment Portfolio [Member] | Derivative Maturity Over Two And Within Three Years From Balance Sheet Date [Member] | Interest Rate Swap [Member] | Short [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 575,000		$ 575,000
Derivative, Average Variable Interest Rate	1.44%		1.44%
Derivative, Average Fixed Interest Rate	0.654%		0.329%
Derivative, Average Remaining Period Until Maturity	2.39		2.89
Interest Rate Risk Associated with the Investment Portfolio [Member] | Derivative Maturity Over Three And Within Four Years From Balance Sheet Date [Member] | Interest Rate Swap [Member] | Short [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 500,000
Derivative, Average Variable Interest Rate	1.042%
Derivative, Average Fixed Interest Rate	0.635%
Derivative, Average Remaining Period Until Maturity	2.56
Interest Rate Risk Associated with the Investment Portfolio [Member] | Derivative Maturity Over Four Years From Balance Sheet Date [Member] | Interest Rate Swap [Member] | Short [Member]
Derivative [Line Items]
Derivative, Notional Amount	$ 1,989,000		$ 2,589,000
Derivative, Average Variable Interest Rate	2.07%		2.301%
Derivative, Average Fixed Interest Rate	0.646%		0.453%
Derivative, Average Remaining Period Until Maturity	6.06		7.00